<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var num = parseInt(prompt('请输入一个数', '4'));
    var sum = 0;
    for (var i = 1; i <= num; i++) {
      if (num % i === 0) {
        sum++;
      }
    }
    if (sum === 2) {
      alert(num + '是一个质数');
    } else {
      alert(num + '不是一个质数')
    }

  </script>
</body>

</html>